Fluid flow meter diagnostics

ABSTRACT

A fluid flow analysis method and corresponding apparatus comprise performing two different sets of diagnostic checks which are associated with different types of flow meters, and cross-referencing the two sets of checks.

RELATED APPLICATIONS

This application is a U.S. national stage entry under 35 U.S.C. § 371from International PCT application Ser. No. PCT/US2016/029399 filed Apr.26, 2016, and entitled “Fluid Flow Meter Diagnostics,” which itselfclaims priority to and is entitled to the filing date of U.S.Provisional application Ser. No. 62/153,490 filed Apr. 27, 2015, andentitled “Fluid Flow Meter Diagnostics.” The contents of theaforementioned applications are incorporated herein by reference.

INCORPORATION BY REFERENCE

Applicant(s) hereby incorporate herein by reference any and all patentsand published patent applications cited or referred to in thisapplication.

TECHNICAL FIELD

The present disclosure relates to fluid flow meter diagnostics, and inparticular to methods of analysing fluid flow, fluid metering apparatus,and corresponding computer program products and methods.

BACKGROUND

Two significant advances in flow meter capabilities over the last twodecades have been the reduction in flow measurement uncertainty andadvances in flow meter diagnostics. These two issues are directlyrelated. A meter's flow rate prediction uncertainty rating is only trulyvalid if there is a guarantee that the meter is fully serviceable. Thisguarantee can be supplied from the meter's diagnostic tools (ordiagnostic ‘suite’).

It could be reasonably argued that over the last twenty years theadvance in flow meter diagnostic suites has been more industriallysignificant than the reduction of flow rate prediction uncertainty.However, no flow meter has a diagnostic suite capable of identifying all(or even most) problems before those problems cause a flow rateprediction bias in excess of the stated uncertainty rating. Industrywill therefore benefit from further advances in flow meter diagnosticcapabilities.

The majority of single phase flow meter designs on the market today usesome single specific fundamental flow metering technique. For example,Differential Pressure (DP) meters cross reference the physical laws ofthe conservation of mass and energy to derive the fluid flow rate. Thevortex meter reads the frequency of shedding vortices off a bluff body.It relates this shedding frequency to the average fluid velocity andtherefore flow rate. The ultrasonic meter takes discrete averagevelocity readings based on the difference in time of flight ofultrasonic waves upstream and downstream along paths in the meter body.With these readings it determines the overall average fluid velocity andhence the fluid flow rate. Much of the present marketing of each meterdesign consists of promoting the pros and downplaying the cons of thatparticular design while doing the opposite for the alternativetechnologies.

Each flow meter design persists on the market as it has benefits forparticular applications. In many flow meter applications differenttechnologies will each successfully and satisfactorily meter the fluidflow. Often there is little performance difference between competingflow meter technologies.

One flow meter design principle is to dispense with the limiting conceptthat one physical metering principle is better than another. It is knownthat there are merits in different physical principles. Therefore,instead of choosing one over the other, there is a principle ofcombining two or more physical principles. Such a hybrid meter shouldhave the combined pros of both meters while potentially negating some ofthe cons of each meter.

Examples of this metering concept are the ideas of Boden (U.S. Pat. No.2,772,567), Pfrehm (U.S. Pat. No. 3,430,489), Lisi (U.S. Pat. No.3,785,204) and Mottram (GB 2,161,941). Boden combined a turbine meterwith a Venturi DP meter. Pfrehm improved Boden's design. Lisi usedBoden's principle to combine a vortex meter with different DP meters.Mottram improved on Lisi's design. These hybrid designs:

-   -   produced metering systems with redundancy—i.e. two meters in        one,    -   produced ‘over determination’ of the flow rate—i.e. two flow        rate predictions to check against each other when the two        sub-systems operated independently,    -   allowed the outputs of different flow meters, using different        physical principles, to be cross referenced thereby producing        extra flow information not attainable from either stand alone        meter.

Therefore, it has been known for many years that combining differentflow meter principles can produce distinct advantages for the meteringof fluid flows.

However, even these hybrid meters still do not provide a diagnosticcapability that can identifying all (or even most) problems before thoseproblems cause a flow rate prediction bias in excess of the stateduncertainty rating. Industry will therefore benefit from furtheradvances in flow meter diagnostic capabilities.

SUMMARY

According to a first aspect of the disclosure there is provided a methodof analysing fluid flow comprising:

-   -   performing a first set of diagnostic checks associated with a        first type of flow meter;    -   performing a second set of diagnostic checks associated with a        second type of flow meter; and    -   cross-referencing the first and second set of diagnostic checks.

The second type of flow meter is dissimilar from said first type of flowmeter.

Optionally, cross-referencing the first and second set of diagnosticchecks comprises identifying diagnostic alerts that are common betweenthe first and second sets of diagnostic checks.

Optionally, performing a first set of diagnostic checks associated witha first type of flow meter comprises providing a first set of possiblefault conditions; performing a second set of diagnostic checksassociated with a second type of flow meter comprises providing a secondset of possible fault conditions; and cross-referencing the first andsecond set of diagnostic checks comprises identifying possible faultconditions which are common to the first and second sets of possiblefault conditions.

Optionally, the first type of flow meter is an ultrasonic flow meter andthe second type of flow meter is a differential pressure flow meter.

Optionally, the ultrasonic flow meter and differential pressure flowmeter are provided in series.

Optionally, a hybrid ultrasonic and differential pressure meter isprovided.

Optionally, the first type of flow meter is a vortex flow meter and thesecond type of flow meter is a differential pressure flow meter.

Optionally, the vortex meter and the differential pressure meter areprovided in series.

Optionally, a hybrid vortex differential pressure meter is provided.

Optionally, the set of diagnostic checks associated with a differentialpressure meter comprises checking two or more differential pressuremeasurements taken by the meter, or measures derived therefrom.

Optionally, the differential pressure (DP) measurements may be selectedfrom the traditional DP, recovered DP and permanent pressure loss DP,from one or more of the PLR, PRR or RPR ratios or normalised valuesthereof.

Optionally, the differential pressure measurements taken by the metercomprise:

-   -   a total head loss differential pressure taken between a position        upstream of a fluid obstruction and a position downstream of the        fluid obstruction;    -   a traditional differential pressure taken between the position        upstream of the fluid obstruction and an intermediate position        between the upstream and downstream positions; and    -   a recovery differential pressure taken between the intermediate        position and the downstream position.

Optionally, the set of diagnostic checks associated with an ultrasonicflow meter comprises checking one or more of: the path speed of sound,velocity ratios (inclusive of one or more of Profile Factor, Symmetry &Cross Flow factors), transducer gain, path performance, path turbulence,path signal to noise ratio.

Optionally, the set of diagnostic checks associated with a vortex flowmeter comprises checking the vortex shedding signal to the backgroundnoise, checking the repeatability/stability of the vortex shedding,monitoring the amplitude and wave form of the shedding vortices.

According to a second aspect of the disclosure there is provided a flowmetering apparatus comprising a first flow meter and a second flowmeter; and a diagnostic system arranged to perform a first set ofdiagnostic checks associated with a first type of flow meter; perform asecond set of diagnostic checks associated with a second type of flowmeter; and to cross-reference the first and second set of diagnosticchecks.

Optionally, the first flow meter is an ultrasonic flow meter and thesecond flow meter is a differential pressure flow meter.

Optionally, the ultrasonic pressure meter and differential pressuremeter are provided in series.

Optionally, a hybrid ultrasonic and differential pressure meter isprovided.

Optionally, the differential pressure meter comprises a Venturiconstriction and the ultrasonic meter comprises at least one transducerpair provided at a throat portion of the Venturi constriction.

Optionally, the first flow meter is a vortex flow meter and the secondflow meter is a differential pressure flow meter.

Optionally, the vortex meter and the differential pressure meter areprovided in series.

Optionally, a hybrid vortex differential pressure meter is provided.

Optionally, the differential pressure meter comprises a Venturiconstriction and the vortex flow meter comprises a vortex shedding bluffbody at a throat portion of the Venturi constriction.

Optionally, the differential pressure meter comprises a fluidobstruction element placed in a fluid conduit; and a support member forthe fluid obstruction element is also a vortex shedding bluff body.

Optionally, the fluid obstruction element comprises a cone-shapedelement comprising a pair of frusto-conical portions joined at theirlarger ends.

Optionally, the support member extends across the entire diameter of afluid conduit.

Optionally, a vortex shedding sensor system is positioned downstream ofthe support member but upstream of a nose portion of the fluidobstruction element.

Optionally, pressure ports for measuring pressures are angularly spacedfrom a vortex sensor port at the conduit.

Optionally, a fluid conduit is provided through the support member for apressure port at one end and for a vortex sensor at its opposite end.

According to a third aspect of the present disclosure there is provideda computer program product including instructions that when executed ona computer device causes it to carry out calculations required for themethod of the first aspect.

Optionally, the device comprises hardware or software included as partof a metering device, a flow computer, general purpose computer, or acombination of one or more thereof.

According to a fourth aspect of the present disclosure there is provideda hybrid flow meter comprising:

-   -   a cone-shaped fluid obstruction element placed in a fluid        conduit and comprising a pair of frusto-conical portions joined        at their larger ends;    -   two or more pressure ports providing measurement points for        measuring a change in fluid pressure caused by the cone-shaped        fluid obstruction element;    -   a support member for the cone-shaped fluid obstruction element        being shaped to function as a vortex shedding bluff body; and    -   a vortex sensor system providing a measurement point for        measuring a vortex shedding frequency generated by the support        member; wherein    -   the support member extends across the entire diameter of a fluid        conduit.

Optionally, the vortex sensor system is positioned downstream of thesupport member but upstream of a nose portion of the cone-shaped fluidobstruction element.

Optionally, the pressure ports are angularly spaced from the vortexsensor port.

Optionally, a fluid conduit is provided through the support member for apressure port at one end and for a vortex sensor port at its oppositeend.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 shows a Venturi meter with instrumentation sketch and pressurefluctuation graph;

FIG. 2 shows a Normalized Diagnostic Box (NDB) with diagnostic results;

FIG. 3 shows a Generic Ultrasonic Meter with a Single Path Shown;

FIG. 4 shows some Typical Path Arrangements for Ultrasonic Meters;

FIG. 5 shows a Sketch of Fully Developed Velocity Profile & Correct PathVelocity Ratios for a chordal four path ultrasonic meter;

FIG. 6 shows an arbitrarily chosen Sample Ultrasonic Meter DiagnosticSuit System/Operator Interface Display;

FIG. 7 shows the Principle of cyclic vortex shedding from a bluff body;

FIG. 8 shows an idealised Vortex Meter Primary Shedding Frequency & theBackground Noise;

FIG. 9 shows an Ultrasonic Meter Embedded in a Venturi Meter Throat;

FIG. 10 shows a Vortex Meter Imbedded in Venturi Meter Throat;

FIG. 11 shows a hybrid vortex and cone meter design;

FIG. 12 shows a Differential Pressure Meter Diagnostic Suite Response toAsymmetrical Flow;

FIG. 13 shows an Ultrasonic Meter Diagnostic Suite Response toAsymmetrical Flow;

FIG. 14 shows a Differential Pressure Diagnostic Suite Response to WetGas Flow;

FIG. 15 shows an Ultrasonic Meter Diagnostic Result Caused by Wet GasFlow;

FIG. 16 shows a DP Meter Gas Entrainment in Liquid Flow DiagnosticResult; and

FIG. 17 shows a Vortex Meter Abnormal Signal Reading for DiagnosticAnalysis.

DETAILED DESCRIPTION

The present disclosure applies to flow of fluid through conduits orchannels. A conduit generally encloses a fluid flowing through it whilea channel will have one or more open sides for the passage of fluidflow. Conduits and channels may be natural or man-made. The term “fluid”as used herein refers to liquids, gases other media, or combinationsthereof. The “other media” may include particulate or solid matter thatis entrained with or dispersed in flowing fluid.

The metering of fluid flow involves measuring a given physicalcharacteristic of fluid flow such as mass flow rate, volume flow rate,average flow velocity, fluid composition, density, temperature orgenerally any other physical parameter that it is desired to monitor.

Flow meters operate according to different physical principles but inall cases include some type of metering apparatus that interacts withthe fluid flow in order to take a measurement. The measurement apparatusmay include an element causing an obstruction to the fluid flow thatcauses a change in pressure and flow velocity together withinstrumentation for measuring pressure at one or more points along aconduit. Examples of this type of apparatus include differentialpressure meters (including Venturi, nozzle, orifice plate and cone typedifferential pressure meters). The measurement apparatus may includetransmitter and/or receivers for sending a signal through the fluid.Examples of this type of apparatus include ultrasonic flow meters whichmeasure the difference between an upstream and downstream time of flightof a sound wave traversing the conduit. Other types of meteringapparatus are discussed elsewhere in this disclosure and known to thoseskilled in the art.

The metering apparatus measures one or more physical parameters (such asdifferential pressure, temperature, time of flight and so on). Thesephysical parameters are used to derive the physical characteristicswhich are being measured, either by direct measurement, or by performingsome further calculations, sometimes with reference to external datasources or other metering apparatus.

The basic concept of flow meter diagnostics is to provide some kind ofcheck to test whether the measured or derived physical parameters areaccurate. Advanced diagnostic systems can also suggest the possiblecauses of an erroneous reading, although this type of capability isgenerally not provided. Diagnostics may be provided by controlling theexisting metering apparatus including external references whereprovided, and may also in some cases use additional diagnostic apparatuseither alone or in combination with the existing metering apparatusand/or references.

In general, a flow meter diagnostic check will involve checking themeasured or derived physical parameters by making redundant readings orby corroborating them with external references and/or expected results.Each different type of flow meter has an associated set of diagnosticchecks that may be provided or carried out. This set of diagnosticchecks is provided as a “diagnostics suite” that includes software thatperforms calculations on measured values and presents results via anassociated display to a user. A diagnostic method therefore includesoperating metering apparatus and any external references and specialiseddiagnostic apparatus according to a set routine to gather specificmeasurements, and then carry out calculations on the measured parametersin order to provide diagnostic information. A diagnostic systemtherefore includes a combination of hardware and software. The softwarecontrols the hardware to implement a metering and diagnostic method.

The diagnostic checks are specific for each type of flow meter and usedexclusively in that context. Each meter design's diagnostic suite alsohas pros and cons. Each meter design's diagnostic suite can see someproblems, and cannot see other problems, before a significant flow rateprediction bias is induced. Each meter design's respective diagnosticsuites are inherently based on the physical principles that that meterutilises. Here, a flow meter “type” is defined by the physical mechanismused to meter the flow. Dissimilar meters use dissimilar physicalmechanisms.

Specialised hardware and or software may be provided for carrying outthe diagnostic checks. This can be provided arbitrary locations,including dedicated hardware or software provided within the body of adevice forming part of the measurement apparatus (one example being the“head” of a DP transmitter used for calculating differential pressuremeasurements), as part of a dedicated flow computer (being a specialruggedized type of computing device for use in the field andspecifically tailored for flow metering applications), as part of ageneral purpose computer, server or other computing device; or indeedany combination of these.

The disclosure in a general sense provides for the combination of a setof diagnostic checks associated with one type of flow meter togetherwith a second set of diagnostic checks associated with a seconddissimilar type of flow meter. These two sets of diagnostic checks arecross-referenced with each other to identify a metering malfunctionand/or to provide significant additional information about the possiblecauses of a metering malfunction, as compared with the information thatcan be derived from a single set of diagnostic checks that is associatedwith a single type of flow meter. Combining dissimilar flow metersinclusive of their respective diagnostic suites produces a more powerfuldiagnostic capability. Some flow meter problems that say a first meter'sdiagnostic suite struggles to see could be much easier identified by asecond meter's diagnostics suite, and vice versa.

Each set of diagnostic checks preferably also provides a set of possiblefault conditions, and a selection of possible fault conditions can beidentified based on the specific results of one or more diagnosticchecks. In a preferred embodiment, a first set of possible faultconditions provided by a first set of diagnostic checks can becross-referenced with a second set of possible fault conditionsassociated with a second set of diagnostic checks, with any possiblefault conditions in common being identified as being good candidates forfurther investigation. This new capability provides maintenance crews orother parties additional intelligence to save time and resourcesinvestigating the causes of faulty metering.

Furthermore, any individual flow meter design's diagnostic suitepresently produces a diagnostic output pattern that its manufacturersand operators attempt to understand. Much development has occurred inthis field but the technology is still relatively immature. Severaldifferent problems can cause any given meter's diagnostic suite toproduce a similar diagnostic output pattern. Therefore, whereas flowmeter diagnostic suites are generally good at correctly telling itsoperator that something is wrong (i.e. giving a generic alarm), they arefar less capable of suggesting just what the particular problem is. Theycan often discount some problems, as those problems are known to producea different diagnostic response. However, they usually have a list ofpotential problems that could cause that particular diagnostic outputpattern. This is rather unsatisfactory. More resolution on the meaningof the diagnostic output is desired.

Combining two different meter designs diagnostic systems can provideadditional information about the meaning of these diagnostic outputs,helping narrow down the list of possible causes for an error or identifya specific cause, and can also provide the means to identify an errorpromptly. A communal metering problem will produce two distinctdiagnostic outputs for the two meters, and different ‘short lists’ ofwhat the problem may be. Only the problems that appear on both shortlists are possible. In some cases the specific problem can beidentified, in others the list of possible problems is significantlyreduced, thereby aiding maintenance crews significantly.

There is potential benefit in combining any two dissimilar flow meters'diagnostic outputs. As way of examples in this text we will discussbelow the particular combinations of ultrasonic meters and DP meterdiagnostic technologies and vortex meter and DP meter diagnostictechnologies.

The disclosure also provides a composite metering device comprising twoor more flow meters of dissimilar type being provided together withcross-referenced diagnostics. The flowmeters of dissimilar type may beconnected in series or parallel, or combined in a hybrid design in whichmetering apparatus associated with a first type of meter and meteringapparatus associated with a second type of meter are either physicallycombined or provided at the same conduit position.

1. DP Meters and DP Meter Diagnostics

Most generic Differential Pressure (DP) meters operate according to thesame principles. FIG. 1 shows a generic sketch of a Venturi DP meter.The inlet pressure (P₁) is measured (shown) along with the fluidtemperature (not shown). From known fluid properties the fluid density(ρ) can be predicted and is then available for use in the flow ratecalculation. This fluid density prediction is therefore external to themetering system.

The generic DP meter utilises the conservation of mass. For anincompressible flow this is expressed as equation 1, where ‘m’ is themass flow rate, ‘Q’ is the volume flow rate, A₁ & A_(t) are the inletand minimum (or ‘throat’) cross sectional areas respectively, and U₁ &U_(t) denote the average fluid velocity at the inlet and throat sectionsrespectively. Rearranging (for an incompressible fluid) gives the inletvelocity expression shown as equation 2.Conservation of mass expression: m=ρQ=ρA₁U₁=ρA_(t)U_(t)  (1)

Re-arranging the conservation of mass expression:

$\begin{matrix}{U_{1} = {\frac{A_{t}}{A_{1}}U_{t}}} & (2)\end{matrix}$

The conservation of energy of a horizontal incompressible flow (with noenergy losses) through a Venturi meter is expressed by equation 3.Rearranging (for an incompressible fluid) gives the inlet velocityexpression shown as equation 4.

Conservation of energy expression:

$\begin{matrix}{{\frac{P_{1}}{\rho} + \frac{U_{1}^{2}}{2}} = {\frac{P_{t}}{\rho} + \frac{U_{t}^{2}}{2}}} & (3)\end{matrix}$

Re-arranging the conservation of energy expression:

$\begin{matrix}{U_{1} = \sqrt{U_{t}^{2} - \frac{2\;\Delta\; P_{t}}{\rho}}} & (4)\end{matrix}$

Note that the differential pressure between the inlet and the throat isthe ‘traditional’ DP and is denoted by ΔP_(t).

$\begin{matrix}{U_{1} = {{\frac{A_{t}}{A_{1}}U_{t}} = \sqrt{U_{t}^{2} - \frac{2\;\Delta\; P_{t}}{\rho}}}} & (5) \\{U_{t} = \sqrt{2\;\Delta\;{P_{t}/\left\{ {\rho\left( {1 - \left( {A_{t}/A_{1}} \right)^{2}} \right)} \right\}}}} & (6)\end{matrix}$

Equating the throat velocity expression from the conservation of mass &energy expressions gives Equation 5. Re-arranging gives Equation 6.Substituting equation 6 into equation 1 produces the mass flowexpression equation 7.

$\begin{matrix}{m = {{\rho\; Q} = {{\rho\; A_{t}U_{t}} = {{\rho\; A_{t}\sqrt{\frac{2\;\Delta\; P_{t}}{\rho\left\{ {1 - \left( \frac{A_{t}}{A_{1}} \right)^{2}} \right\}}}} = {{EA}_{t}\sqrt{2\;\rho\;\Delta\; P_{t}}}}}}} & (7)\end{matrix}$

Note that beta (β), a geometry value for a given DP meter, is defined byequation 8. The “Velocity of Approach” (denoted as ‘E’), a geometryvalue, is defined as equation 9. In reality the discharge coefficient(C_(d)) is added to account for real world effects, while an expansionfactor (ε) is used in gas flow meter applications to account for the gasdensity fluctuation through the DP meter body. The actual mass flow rateprediction equation for a generic DP meter is therefore equation 10. Theactual volume flow rate prediction is equation 11

$\begin{matrix}{\beta = \sqrt{\frac{A_{t}}{A_{1}}}} & (8) \\{E = \frac{1}{\sqrt{1 - \beta^{4}}}} & (9) \\\begin{matrix}{m = {{\rho\; Q} = {{EA}_{t}ɛ\; C_{d}\sqrt{2\;\rho\;\Delta\; P_{t}}}}}\end{matrix} & (10) \\\begin{matrix}{Q = {{EA}_{t}ɛ\; C_{d}\sqrt{\frac{2\;\Delta\; P_{t}}{\rho}}}}\end{matrix} & (11)\end{matrix}$

Steven (U.S. Pat. No. 8,136,414) developed a generic DP meter diagnosticsuite. FIG. 1 shows a sketch of a Venturi DP meter and its pressurefield. The DP meter has a third pressure tap downstream of the twotraditional pressure ports. This allows three DPs to be read, i.e. thetraditional (ΔP_(t)), recovered (ΔP_(r)) and permanent pressure loss(ΔP_(PPL)) DPs. These DPs are related by equation 12. The percentagedifference between the inferred traditional DP (i.e. the sum of therecovered & PPL DPs) and the read DP is δ %, while the maximum alloweddifference is θ%.DP Summation: ΔP _(t) =ΔP _(r) +ΔP _(PPL), uncertainty±θ%  (12)Traditional flow calculation: m _(trad) =f _(t)(ΔP _(t)), uncertainty±x%  (13)Expansion flow calculation: m _(exp) =f _(r)(ΔP_(r)), uncertainty±y%  (14)PPL flow calculation: m _(PPL) =f _(PPL)(ΔP _(PPL)), uncertainty±z%  (15)

Each DP can be used to meter the flow rate, as shown in equations 13, 14& 15. Here m_(trad), m_(exp) & m_(PPL) are the mass flow ratepredictions of the traditional, expansion & PPL flow rate calculations.Symbols f_(t), f_(r) & f_(PPL) represent the traditional, expansion &PPL flow rate calculations respectively, and, x %, y % & z % representthe uncertainties of each of these flow rate predictions respectively.Inter-comparison of these flow rate predictions produces threediagnostic checks. The percentage difference of the PPL to traditionalflow rate calculations is denoted as ψ%. The allowable difference is theroot mean square of the PPL & traditional meter uncertainties, ϕ%. Thepercentage difference of the expansion to traditional flow ratecalculations is denoted as λ%. The allowable difference is the root meansquare of the expansion & traditional meter uncertainties, ξ%. Thepercentage difference of the expansion to PPL flow rate calculations isdenoted as χ%. The allowable difference is the root mean square of theexpansion & PPL meter uncertainties, ν%.

Reading these three DPs produces three DP ratios, the ‘PLR’ (i.e. thePPL to traditional DP ratio), the PRR (i.e. the recovered to traditionalDP ratio), the RPR (i.e. the recovered to PPL DP ratio). DP meters havepredictable DP ratios. Therefore, comparison of each read to expected DPratio produces three diagnostic checks. The percentage difference of theread to expected PLR is denoted as α%. The allowable difference is theexpected PLR uncertainty, a %. The percentage difference of the read toexpected PRR is denoted as γ%. The allowable difference is the expectedRPR uncertainty, b %. The percentage difference of the read to expectedRPR is denoted as η%. The allowable difference is the expected RPRuncertainty, c %.

These seven diagnostic results can be shown on the operator interface asplots on a graph. That is, we can plot (FIG. 2) the following fourco-ordinates to represent the seven diagnostic checks:(ψ%/ϕ%, α%/a %), (λ%/ξ%, γ%/b %), (χ%/ν%, η%/c %)&(δ%/θ%, 0).

For simplicity we can refer to these points as (x₁,y₁), (x₂,y₂), (x₃,y₃)& (x₄,0).

The act of dividing the seven raw diagnostic outputs by their respectiveallowed variances is called ‘normalisation’. A Normalised DiagnosticsBox (or ‘NDB’) of corner coordinates (1,1), (1,−1), (−1,−1) & (−1,1) canbe plotted on the same graph (see FIG. 2). This is the standard userinterface with the DP meter diagnostic system. All four diagnosticpoints inside the NDB indicate a serviceable DP meter. One or morepoints outside the NDB indicates a meter malfunction.

2. Transit Time Ultrasonic Meters and Ultrasonic Meter Diagnostics

An ultrasonic wave moving downstream or upstream in a homogenous fluidflow moves at the speed of sound plus or minus the fluid velocityrespectively. Hence, if the difference between the upstream anddownstream time of flight (Δt) between two points (of known distanceapart) is measured, this will give the average fluid velocity along thatpath (u).

Consider the meter geometry shown in FIG. 3. The left side shows across-sectional view of a fluid conduit looking down the longitudinalaxis of the conduit. Ultrasound transceivers are placed at positions aand b. These positions are in a plane parallel to the sagittal plane ofthe fluid conduit (left hand side figure), but are at differentlongitudinal positions, as seen in the right hand side of the figure.

Across the path shown the upstream (t_(ab)) and downstream (t_(ba))transit times are calculated by equations 16 & 17 respectively. Notethat ‘c’ and ‘u’ are the speed of sound and the average velocity acrossthat path.

$\begin{matrix}{t_{ab} = \frac{L}{c - {u\;\cos\;\theta}}} & (16) \\{t_{ba} = \frac{L}{c + {u\;\cos\;\theta}}} & (17)\end{matrix}$

Equations 16 & 17 are solved for the average velocity, i.e. see equation18.

$\begin{matrix}{u = {{\frac{L}{2\;\cos\;\theta}\frac{\Delta\; t}{t_{ab}t_{ba}}} = {\frac{L^{2}}{2d}\frac{\Delta\; t}{t_{ab}t_{ba}}}}} & (18)\end{matrix}$

It can also be shown that the speed of sound is found by equation 19.

$\begin{matrix}{c = \frac{L\;\Delta\; t}{2t_{ab}t_{ba}}} & (19)\end{matrix}$

If we were to consider the average velocity (u) measured across thesingle path representative of the average velocity in the pipe (u_(αν)),then from equation 1, the volume flow is given by equation 20 and themass flow is given by equation 21.

$\begin{matrix}{Q = {{A_{1}u_{av}} = {A_{1}\frac{L^{2}}{2d}\frac{\Delta\; t}{t_{ab}t_{ba}}}}} & (20) \\{m = {{\rho\; Q} = {\rho\; A_{1}\frac{L^{2}}{2d}\frac{\Delta\; t}{t_{ab}t_{ba}}}}} & (21)\end{matrix}$

In reality, all fluid flows in pipes have a velocity profile, where dueto wall friction the local velocity varies across the cross sectionalarea, from lowest velocity near the wall to highest at the centreline(see FIG. 5). Hence, the calculation of the true average velocity of theflow (u_(αν)) is dependent on the number of paths and where these pathsare located.

FIG. 4 shows typical path arrangements for ultrasonic meters. The methodof averaging the individual path velocity measurements to obtain theoverall the average flow velocity (u_(αν)) is given by the followingexpressions, for cases A, B, C and D respectively:

u_(av) = u  $u_{av} = \frac{u_{1} + u_{2}}{2}$$u_{av} = \frac{u_{1} + u_{2}}{2}$$u_{av} = \frac{\sum{w_{i}V_{i}}}{A_{1}}$

One commercially common path configuration is the four path design (FIG.4D). The calculation relating the four individual discrete pathvelocities to the average flow velocity and volume flow rate is morecomplicated in this configuration. These chordal (i.e. path) locationsare set by techniques for numerical integration. (V_(i) representsvolume flow and w_(i), represents a weighting fraction derived for aparticular geometry, i.e. chordal positions).

Regardless of the number of paths used, and whatever the chordalpositions of those paths are, an ultrasonic meter will predict theaverage fluid velocity through the meter. Hence, all ultrasonic meterspredict the fluid volume flow by finding the average velocity (u_(αν))and then applying equation 22. Unlike the DP meter the ultrasonic metercan predict the fluids volume flow rate without knowledge of the fluiddensity. However, as with the DP meter, the ultrasonic meter cannot tellthe mass flow rate without an external input of the fluid density.Equation 23 is the ultrasonic meter mass flow rate equation.Q=A₁u  (22)m=ρA₁u  (23)

Unlike DP meters all commercial ultrasonic meters have had a genericdiagnostic system available since the early development phase of themetering concept. There tends to be several diagnostic checks that mostultrasonic meter manufacturers utilise, including:

-   -   Path Speed of Sound    -   Velocity Ratios (including checks on various derived parameters)    -   Transducer Gain    -   Path Performance (i.e. signal quality)    -   Path Turbulence (i.e. standard deviation of signals)    -   Path Signal to Noise Ratio        Speed of Sound

Each ultrasonic path measures the average velocity across that path andthe speed of sound (see equation 19). The speed of sound measurement isindependent of the fluid velocity. Hence, for a homogenous fluid flow,each path should measure the same speed of sound. A diagnostic check isto compare the speed of sound readings of each path. They have to agreewithin an allowable uncertainty. For multi-path ultrasonic meters, ifthe speed of sound measurements do not agree a diagnostic alarm is set.If one path's speed of sound is significantly different to severalothers this suggests that that path has a problem.

Furthermore, if the individual path speed of sound measurements doagree, then their average speed of sound can be compared to a speed ofsound check independent of the meter. This independent check is usuallycalculated from fluid samples, pressure & temperature readings and areputable Equation of State calculation. If the meter and externalsupplied speed of sound readings do not match this is a diagnosticresult suggesting that the external speed of sound prediction isincorrect. The practical implication of that is the fluid propertiesbeing used are likely to be incorrect, and hence the fluid density andtherefore the mass flow rate predictions may be wrong.

Velocity Ratios

The velocity ratio is defined as the ratio of an individual path'saverage velocity to the average measured flow velocity. For a ‘fullydeveloped flow’, i.e. flow with no upstream disturbances, velocitydistribution across a pipe/meter body is symmetrical (see FIG. 5) andhas no swirl component. A given ultrasonic meter geometry, with setpaths, should therefore have set predictable velocity ratios. Theaverage flow velocity can increase or decrease, but the velocity ratiosremain relatively constant over a very large flow range. Velocity ratiosare therefore a useful ultrasonic meter diagnostic tool.

Industry usually looks at a plot similar to the right hand side of FIG.5, but also uses basic mathematical relationships to create parametersthat more clearly show any velocity ratio diagnostic patterns. Threecommon parameters based on velocity ratios are the Profile Factor(‘PF’), symmetry (‘S’) and cross flow (‘CF’). Profile Factor, symmetryand cross flow are defined by equations 24, 25 & 26 respectively.Symmetry and Cross Flow should theoretically be unity (and are set atsome value close to unity by calibration). The Profile Factor is set bythe path geometry and calibration.

$\begin{matrix}{{P\; F} = \frac{\left( {u_{2}/u_{av}} \right) + \left( {u_{3}/u_{av}} \right)}{\left( {u_{1}/u_{av}} \right) + \left( {u_{4}/u_{av}} \right)}} & (24) \\{{S = \frac{\left( {u_{1}/u_{av}} \right) + \left( {u_{2}/u_{av}} \right)}{\left( {u_{3}/u_{av}} \right) + \left( {u_{4}/u_{av}} \right)}},} & (25) \\{{C\; F} = \frac{\left( {u_{1}/u_{av}} \right) + \left( {u_{3}/u_{av}} \right)}{\left( {u_{2}/u_{av}} \right) + \left( {u_{4}/u_{av}} \right)}} & (26)\end{matrix}$

Velocity ratios, and these associated parameters, can indicate if theflow is asymmetrical and/or swirling (which causes flow rate predictionbiases) or if one (or more) path/s have a problem (which leads toincorrect path velocity measurement and hence flow rate predictionerrors). An allowable variation/uncertainty is set by the operator foreach parameter, and an alarm is set if this is exceeded. Other pathconfigurations can have similar analysis with the precise parametersbeing chosen for those configurations.

Transducer Gain

Each chord has two ultrasonic transducers, transducers A & B. Thesetransducers send ultrasonic wave signals through the flowing medium toeach other. In normal operation the strengths of the signals sentupstream & downstream remain relatively constant. Modern ultrasonicmeters have an automatic ‘gain control’, i.e. an automatic control onsignal strength required in order that that signal can be identified byits paired transducer. As there are two transducers per chord there aretwo gain (dB) values associated with each chord. An increase intransducer gain (i.e. required signal strength) is the meter reacting toa weaker signal at the receiving transducer. This can be caused byvarious issues, e.g. transducer malfunction, contamination on thetransducer face, liquids in a gas flow etc. An allowablevariation/uncertainty is set by the operator for each transducers gainvalue (noting there are two per chord—A to B & B to A), and an alarm isset if this is exceeded.

Path Performance

This ultrasonic meter diagnostic checks the performance of an individualpath. Each path attempts to measure a set number of delta time readingsper unit time. If one of those attempted measurements was unsuccessful(for whatever reason, e.g. the ultrasonic wave signal was too weak,distorted etc.) then that difference in time measurement is lost. Theratio of the successfully read to attempted delta time measurements isthat paths ‘performance’. Monitoring this ‘performance’ is anotherultrasonic meter diagnostic check. Correct operation of a path shouldproduce a high performance. However, the path performance can beaffected by a malfunctioning transducer, by flow disturbance or otherproblems. An allowable variation/uncertainty is set by the operator foreach path performance, and an alarm is set if this is exceeded.

Path “Turbulence”

The term ‘turbulence’ in ultrasonic meter diagnostics should not beconfused with the meaning of ‘turbulence’ in fluid mechanics. The‘turbulence’ value of an ultrasonic meter's path is the standarddeviation of that path's delta time readings, i.e. the stability of theresult over time. Correct operation of a transducer pair with areasonably stable flow should produce a relatively low repeatablestandard deviation. The outside paths, i.e. the paths closer to themeter wall, tend to see more turbulence than the inside paths due towall friction effects. However, any given individual path will have somereasonably constant signal standard deviation. If any path startsshowing a change in this standard deviation this could be an indicationof a metering problem, e.g. a problem with a transducer, or a change inthe stability of the flow. For example, a partial blocked flowconditioner upstream of the ultrasonic meter causes significant flowdisturbance which will manifest itself in a flow rate prediction biasand a significant increase in path standard deviation. An allowablevariation/uncertainty is set by the operator for each paths‘turbulence’/‘standard deviation’, and an alarm is set if this isexceeded.

Path Signal to Noise Ratio (SNR)

Each transducer can read the background acoustic noise between sendingand receiving signals from its paired transducer. This allows it todistinguish between the background noise and the signal. A correctlyoperating transducer in a normal flow metering application should see avery significant difference between the strength of the signal and thebackground noise. The measure of signal strength to background noise iscalled the “Signal to Noise Ratio”, or “SNR”.

There can be quite a range of SNR's between meter applications. As longas the signal can be distinguished from the background noise, a low SNRis not in itself a problem. However, monitoring the SNR can be a usefuldiagnostic. An unusually low SNR (i.e. excessive noise) can indicate thepresence of a noise generating pipe component close to the meter. Ifthat noise generation is not controlled it could increase to cause themeter to fail. An allowable minimum SNR value is set by the operator foreach path, and an alarm is set if this is exceeded.

Wave Form

A newer ultrasonic meter diagnostic is the monitoring of wave form. Eachtransducer sends a signal by vibrating in the flowing medium to createultrasonic waves. The vibration ramps up and ramps down causing theamplitude of the waves of each signal to increase to a maximum anddecrease away from that maximum. The form of the signals resulting waveform should be clearly decipherable at the receiving transducer. Thelevel of deformation of the received wave form holds information to thecondition of the flowing medium in which it passed.

Ultrasonic Meter Diagnostic Displays

Most ultrasonic meter manufacturers have chosen their own ways ofdisplaying these diagnostics checks on their meter/operator interface.There is no agreed standard for display. FIG. 6 shows an example of onemanufacturer's chosen ultrasonic meter diagnostic display. As shown, thedisplay has separate portions which show the path velocity ratios, pathspeed of sound checks, path performance checks, transducer gain settingchecks, transducer SNR checks, path turbulence checks, velocity ratioparameters, and diagnostic output text. This is shown as an example onlyand the invention disclosed here is not ultrasonic meter or display typespecific.

Regardless of the display lay out, these ultrasonic meter diagnosticchecks are valuable for telling the operator information about both thestate of the meter and the flow through the meter.

3. Vortex Meters and Vortex Meter Diagnostics

A vortex meter operates by exposing a bluff body to the fluid stream.Vortices are shed from the bluff body in a cyclic fashion (see FIG. 7).This series of downstream vortices is called a “von Karman vortexstreet”. The vortex shedding frequency, as shown as “signal” in FIG. 8,has a nominally linear relationship with the average fluid velocity.Hence, reading the vortex shedding frequency allows the average flowvelocity and therefore the volume flow rate to be found.

The shape of the bluff body is manufacturer dependent. The vortexshedding frequency is read by different manufacturers in different ways,e.g. DP fluctuations between either side of the bar, strain gauges on atab put under cyclic stress by the vortices, fluctuations in ultrasoundwaves downstream of the bluff body etc. The shape of the bluff body andthe choice of vortex shedding frequency is not significant to thegeneral metering principle or the generic diagnostic system beingdiscussed here.

The generic vortex meter volume flow rate and mass flow rate equationsare shown as equation 27 & 28, where ‘f ’ is the read vortex sheddingfrequency and “K_(v)” is the vortex meter “K-factor” which is usuallyfound by calibration.

$\begin{matrix}{\overset{.}{Q} = {{A_{1}U_{1}} = \frac{f}{K_{v}}}} & (27) \\{\overset{.}{m} = {{\rho\; Q} = {{\rho\; A_{1}U_{1}} = {\rho\frac{f}{K_{v}}}}}} & (28)\end{matrix}$

In practice the vortex meter sensor will pick up the primary vortexshedding frequency and background noise. FIG. 8 shows the two separatesignals. In reality the two signals are not received separately (asshown in FIG. 8) but combined, i.e. superimposed on top of each othermaking a more complicated signal reading. In most applications theprimary vortex shedding frequency will be strong, and for a relativelysteady flow, of relatively constant amplitude and wavelength. Backgroundnoise usually consists of a weaker signal of different wavelength, whichmay or may not be steady depending on the source.

Vortex meter manufacturers tend to install a capability to send a testsignal from the sensor to the electronics to confirm that theelectronics are serviceable. If the vortex meter passes this internalcheck then there is a guarantee the problem is not in the sensorsystem/electronics. Any unusual vortex shedding sensor results aretherefore seen to be due to either a meter issue (i.e. corrosion,contamination, erosion/impact damage etc.) or flow condition issues(e.g. bubbly liquid, wet gas, asymmetric flow, pulsation etc.). In thiscase generic vortex meter diagnostics use various forms of sophisticatedsignal analysis to distinguish background noise from the primary signal,and to analyse the primary signal (and the background noise) to decipherwhat information is in the signal. This is a vortex meter type of waveform analysis. From this analysis vortex meter diagnostics candistinguish between a sensor malfunction and real physical issues. Thisanalysis can then discount some physical problems and short list othersthat could cause such a signal abnormality. Vortex meter diagnosticoutput displays are manufacturer dependent, i.e. there is no set displayused throughout industry.

4. A Mass Flow Meter Concept

As mentioned above, Boden and Pfrehm combined a turbine meter with a DPmeter. Lisi and Mottram combined a vortex meter with a DP meter. Turbineand vortex meters both predict the average velocity and hence the volumeflow rate without requiring the fluid density as an input. They aretherefore called ‘velocity meters’, and their volume flow rateprediction is density insensitive. DP meters require the fluid densitybe supplied from an external source in order to meter the volume or massflow rate. That is, their volume flow rate prediction is densitysensitive. Boden et al stated that density insensitive and densitysensitive meters in series would produce a volume flow rate prediction,a density prediction and therefore a mass flow rate prediction. Thismeter combination does not require density information from an externalsource in order to predict the mass flow rate. Such a metering systemwhich predicts mass flow rate without any density information beingsupplied can be called a “mass meter”. Furthermore, the two dissimilarmeters in series offer flow metering redundancy, along with flow rateover determination (if the density value is available from anexternal/independent source) and rudimentary inter-comparisondiagnostics of flow rate and density predictions.

An ultrasonic meter is, like the vortex and turbine meters, a ‘velocitymeter’. It meters the volume flow rate with no density input required.Both ultrasonic and vortex meter's volume flow rate predictions aredensity insensitive. Hence, an ultrasonic meter, vortex meter or anyvelocity meter can be used with a DP to form a mass meter. An ultrasonicmeter, vortex meter or any velocity meter can be installed with a DPmeter independently in series, or a hybrid design can be used. One suchhybrid design is the ultrasonic meter in the throat of the Venturi (ornozzle) meter, as sketched in FIG. 9. Another hybrid design is a vortexmeter in the throat of the Venturi (or nozzle) meter, as sketched inFIG. 10. It is possible to modify the design of the meter of FIG. 10 byangularly spacing the low pressure tap and the vortex sensor port, forexample by 180 degrees, so that they do not interfere with each other.

The type of vortex meter (e.g. bluff body design & shedding sensordesign) or the type of ultrasonic meter (e.g. transducer design, numberand position of paths) is arbitrary. Any pair of stand-alone velocitymeter and DP meter designs could be put in series. Any calibratedVenturi & ultrasonic meter hybrid design, or any calibrated Venturi andvortex meter design, where the two meters operate successfully asindependent meters will operate as a volume meter, densitometer and massflow meter.

The velocity meter, regardless of whether it is a stand-alone meter inseries with a DP meter, or provided as part of a hybrid design such asin the throat of a Venturi or nozzle meter, will produce an independentvolume flow rate prediction (i.e. see equation 22). The DP meter volumeflow rate prediction is shown as equation 11. If we denote the velocitymeter's volume flow rate prediction as Q_(vm), then substituting thatvalue into the DP meter equation gives equation 29.

$\begin{matrix}{Q_{vm} = {{EA}_{t}ɛ\; C_{d}\sqrt{\frac{2\;\Delta\; P_{t}}{\rho}}}} & (29) \\{\rho = {\left( {2\;\Delta\; P_{t}} \right)\left\{ \frac{{EA}_{t}ɛ\; C_{d}}{Q_{vm}} \right\}^{2}}} & \left( {29a} \right)\end{matrix}$

The only unknown parameter in equation 29 is the fluid density. Equation29a shows the calculation re-arranged to find the density. Once thedensity and volume flow rate are known the mass flow is derived fromequation 1.

Such density sensitive and density insensitive flow meter combinationshave been considered periodically for many years. However, the additionof also combining their respective diagnostic suites has not until nowbeen considered.

Presently no meter manufacturer promotes a velocity meter and a DP meterin series, or a hybrid velocity and DP meter, as a mass flow meter. Someultrasonic and vortex meter manufacturers do restrict (i.e. reduce) theinlet area of their meters and then expand the area back to the pipearea behind the meter. Whereas this may look like a Venturi meter withan ultrasonic or vortex meter in the throat it is not. No DPs are readfor flow measurement. The purpose of the reduction of inlet area is toaccelerate the flow and thereby help flow condition the velocity profileto the ‘fully developed’ flow profile required for good metering. Hence,this design is not a Venturi meter with a velocity meter located at thethroat. No DPs are read, no Venturi meter flow rate predictions aremade, and no DP meter diagnostics are considered.

5. A Hybrid Vortex and Cone Differential Pressure Meter

One type of differential pressure meter comprises a fluid obstructionelement placed in a fluid conduit to cause a pressure drop. The fluidobstruction element is held in place by a support member. The fluidobstruction element may be of many different shapes but one example isthe so-called cone DP meter, in which the fluid obstruction elementcomprises a pair of frusto-conical portions joined at their larger ends.An example hybrid design is shown in FIG. 11, which shows thecombination of a vortex meter and a differential pressure meter.

The standard cone DP meter has a circular cone support bar that extendsvertically down from the wall of a conduit to its centre line, where itis attached to the apex of the cone. The basic concept of a hybridvortex and DP meter is to use the support member of the fluidobstruction member as a vortex shedding bluff body. The normal circularprofile of the support bar may be modified in order to provide effectivevortex shedding.

However, the inventor has found that a bluff body/cone support bar thatonly extends down to the centre line of the fluid conduit is noteffective and does not guarantee vortex shedding.

Standard vortex meters have bluff bodies that extend the fulllength/diameter of the pipe. It was realized that in a hybrid design, avortex meter bluff body that only extending half way across a pipe wascreating a flow disturbance due to bluff body tip effects, and thatdisturbance was interfering with the vortex shedding. It was discoveredthat it is not possible to simply replace the cone meter standardsupport with a bluff body support. A hybrid design must have a fulllength vortex meter bluff body for the vortex meter to be guaranteed tooperate successfully.

A hybrid design therefore extends the bluff body/cone support to thefull meter body diameter. The bluff body spans the length of the meterbody diameter and can be attached at both ends to the pipe wall. Thisdesign gave the bluff body more chance of producing the required vortexshedding while it had the additional structural benefit of significantlyincreased the stiffness of the bluff body/cone assembly. This increasesthe structure's resonance frequency thereby reducing the likelihood ofany long term fatigue failure, while also increasing the overallstrength of the assembly.

The inventors have also realised that the position of the vortexshedding sensor is important for vortex shedding to be reliablydetected. Testing has shown that vortex shedding is intermittent, notsteady, and not reliable if the vortex shedding sensor is too close tothe downstream cone element. If the vortex shedding sensor is positionedat the standard downstream location as found with standard stand-alonevortex meters, and the cone element is positioned at the standard conemeter position downstream of the cone support, then it has been foundthat the vortices being produced by the vortex shedding bluff body aredestroyed or dissipated by the cone. At the location of the vortexsensor the vortices varied between weak and non-existent.

Therefore, the inventor has realised that the vortex sensor should bedownstream of the bluff body support bar and yet upstream (and clear of)the nose of the downstream cone. Note that here, the ‘nose’ of the coneis defined as the start of the reduction of flow area (as shown in FIG.11). FIG. 11 shows the hybrid meter prototype design.

A final design consideration born of the practical experience ofmanufacturing these prototypes is that the coupling for the pressureport through the vortex shedding bluff body support bar is in very closelongitudinal proximity to the coupling for the vortex sensor. Combiningthe couplings beside each other means that the welds for the couplingscan overlap. This can cause welding/manufacturing difficulties, andcould, if the welding was compromised, cause structural weakness. It wastherefore realized that the coupling for the vortex sensor could beangularly offset from the coupling for the pressure port; and with theextension of the bluff body support bar across the length of the pipediameter the vortex sensor and pressure port could be a positioneddirectly opposite, at 180 degrees, thereby avoiding sensor mountingcongestion. This is shown in FIG. 11, showing a hybrid design withpressure taps 1100, 1102, vortex sensor 1104, cone 1106, and combinedsupport member and bluff body 1108.

6. Advantages of Combining Flow Meters and Their Respective DiagnosticOutputs

Combining (i.e. cross referencing) the diagnostic systems of differentflow meters in series, or combined in a hybrid metering system, isadvantageous as it supplies the system operator significantly enhanceddiagnostic information.

Some examples of this technique will now be explained. These examples donot limit the scope of the disclosure and do not represent an exhaustivelist of the possibilities. However they do clearly disclose toindividuals skilled in the art of flow metering the wide rangingadvantages such a combination of technologies offers.

EXAMPLE 1

DP meters and ultrasonic meters (like most flow meters) require that theinlet flow is ‘fully developed’, i.e. has no flow disturbances. If theflow is disturbed it affects the relevant sensor readings on bothmeters, and hence causes a flow rate prediction bias. This bias willtraditionally go unnoticed unless meter diagnostics can identify aproblem exists. Both the DP meter and ultrasonic meter diagnostic suitescan identify a problem when there is some disturbance upstream of themeter, say a partially blocked flow conditioner making the problem ofdisturbed flow worse instead of better. However, note that therespective diagnostic suites show ‘a’ problem exists, not ‘thatparticular problem’. If the problem is extreme enough then perhaps theindividual diagnostic suites will independently identify asymmetrical(i.e. disturbed) flow exists. However, if the issue is severe enough,then the issue will become evident by external mass balance checks. Itis important to identify a problem, and ideally what the problem isbefore the issue (and mis-measurement) becomes extreme.

FIG. 12 shows a reproduction of an orifice DP meter diagnostic suiteresponse to disturbed (asymmetrical) flow. The diagnostic result is ofthe form shown in FIG. 2, that is, symbols which out outside thenormalised diagnostics box indicate that there is a problem. Theresponse presents to the operator a list of possible problems which inthis case include asymmetrical flow, partial blocked throat, inletdiameter entry high, and throat diameter entry low. The response ofgeneric DP meters are all similar. Modern DP meter diagnostic suite alsosupplies a ‘short list’ of problems that can cause that diagnosticpattern. This short list shows the known problems that will induce a DPmeter's diagnostic system to show that diagnostic warning pattern. (ForDP meters this averaged pattern is also caused by wet gas flow, but wetgas flow also causes a tell-tale instability in the DP readings, i.e. ahigh standard deviation in the DPs read, and hence wet gas would bediscounted as a possibility by the diagnostic system in this case.)

FIG. 13 shows a hypothetical response of a generic ultrasonic meterdiagnostic suite to asymmetrical flow. The modern ultrasonic meterdiagnostic suites do not yet tend to present the operator with a ‘shortlist’ of potential problems that could cause that diagnostic pattern.Nevertheless, a trained operator could deduce such a list from examiningthe diagnostic pattern.

FIG. 13 shows for a low to moderate asymmetrical flow (where the inducedflow rate error is still enough to be very significant), much of theultrasonic meter diagnostic suite does not pick up the problem. Thespeed of sound check is immune to the problem. The path performance,gain, SNR and turbulence diagnostics are not sensitive enough to see lowto moderate asymmetrical flow. It is the path velocity ratio checks, andspecifically the profile factor, that is sensitive to this problem. Anexperienced engineer could see such a diagnostic response shown in FIG.13 and note that there is a list of ultrasonic meter problems that couldcause such a warning pattern. That list is shown in FIG. 13 andcomprises wet gas flow, asymmetrical flow, failing transducer, and lightcontamination on a transducer face.

Both the ultrasonic meter and DP meter see a problem but they cannotindividually identify what specific problem exists. However, if the twometers are used together, cross referencing the two meters diagnosticsuites cuts the potential problems down very significantly. In this casethe combined system would show the problem is specificallyasymmetric/disturbed inlet flow.

EXAMPLE 2

Wet gas flow is an extremely adverse flow condition for both DP andultrasonic meters. FIG. 14 shows sample DP meter diagnostic data takenfrom a 6″ Venturi meter under wet gas flow tests. The Venturi meter'sdiagnostic suite is known to be very sensitive to wet gas flow. Theresult shown is for liquid flow of <0.5% by total volume. The averagepattern is similar to the DP meter disturbed flow pattern. However, anaccompanying significant increase in DP instability informs the DP meterdiagnostic system that the flow is either wet gas flow or a combinationof another problem coupled with the flow being unsteady, as illustratedby the box on the left hand side of FIG. 14.

As with asymmetrical flow if the problem is severe enough the issue maybe seen by external checks for an ultrasonic meter. However, we areinterested in low to moderate problems, where the diagnostics are neededto identify a problem, and ideally what the problem is, before the issue(and mis-measurement) becomes extreme. The ultrasonic meter diagnosticsuite has a number of checks that can identify wet gas flow, butunfortunately most only see wet gas once the liquid loading is moderateto substantial and the metering error is very significant. The primary(most sensitive) ultrasonic meter diagnostic checks for wet gas flow areagain the path velocity ratios, and in particular the symmetry andprofile factor. For low to moderate liquid loading wet gas flows it isthese diagnostics that will set an alarm, i.e. see FIG. 15.

FIG. 15 shows a list of potential issue that could cause such anultrasonic meter diagnostics suite response. Not surprisingly in thisexample they are the same as in the first example. However, note whencross-referenced with the DP meter diagnostic response it can beinferred that wet gas flow is the communal problem. (Note that theultrasonic meter discrete velocity measurements do not tend to have thesame level of instability with wet gas flow as a DP meter's DP signal.This allows for a distinction.)

EXAMPLE 3

The diagnostic suites of the ultrasonic meter and DP meter togethermutually support each other.

If an ultrasonic meter speed of sound check does not agree with theexternal independent speed of sound prediction then this is anindication that the gas chromatograph/Equation of State fluid propertypredictions are erroneous. A stand-alone DP meter's diagnostic system isblind to erroneous density inputs. In this example, the combination ofthe ultrasonic meter and DP meter diagnostics results in the ultrasonicmeter showing that both meters' density input (from the communalexternal density prediction source) is untrustworthy. In this case theoperator can then utilise the combined ultrasonic meter & DP meterdensity prediction (i.e. see Section 4).

Alternatively, if one or more of the ultrasonic meter transducers beginsto fail in service, the ultrasonic meter diagnostics will indicate thisissue. The DP meter diagnostic suite will be unaffected, and the warningalarm will be from the ultrasonic meter diagnostic suite alone. This isan indicator that the problem cannot be flow conditions (e.g. wet gasflow or asymmetrical flow) as this would affect both meters in their ownways. The problem is therefore known to be with the ultrasonic metermechanics only. Hence, in this case the ultrasonic meter defaults toassuming the velocity across that lost path/s by assuming the velocityprofile was fully developed. That is, the ultrasonic meter has to assumethat there are no flow condition problems that its diagnostic system isblind to while one or more of its transducers are off line. However, theDP meter and its diagnostic system in series with the ultrasonic meterwill show the operator there are no flow condition problems therebyalleviating the need for this risky unfounded assumption. Furthermore,the DP meter with no diagnostic alarm is shown to be fully serviceableand can therefore take over as the primary meter until the ultrasonicmeter receives maintenance.

EXAMPLE 4

Say a liquid flow is being metered with a vortex meter and a DP meter.In this example the liquid has entrained gas. Entrained gas in liquidflows (i.e. ‘bubbly flow’) causes flow meters to mis-measure the liquidflow rate.

FIG. 16 shows the diagnostic result of an orifice meter with gasentrained in the liquid. Gas entrained in liquid flows causes the DPmeter read DPs to be relatively unsteady. This is a direct analogy withwet gas flow in gas flow applications. This DP stability issue helps theDP meter diagnostic system.

FIG. 16 shows an example of an abnormal vortex shedding frequency whichwould be analysed by a vortex meter diagnostic suite. In this case thevortex meter diagnostic suite would first check the integrity of thetransmitter by sending a signal to it. Once the signal was confirmed asreal, signal analysis would short list potential issues that could causesuch a signal. In this example two-phase flow (i.e. gas entrained in theliquid flow meter application) is one communal result between the DP &vortex meter diagnostic systems. The presence of both meters and theirdiagnostic suite therefore significantly reduces the list of possibleproblems the meter may be encountering.

These examples show that instead of being competing independenttechnologies, as industry presently views DP & various velocity meterdesigns and their respective diagnostic suites, this disclosure ofcombining the two technologies inclusive of their respective diagnosticsuites offers significant advantages to industry.

Naturally, those skilled in the art of flow metering will realise thatsimilar benefits are clearly available by combining differentembodiments of this generic concept. Any dissimilar flow meters (i.e.two or more meters that operate by utilising different principles, i.e.different laws of physics) with respective diagnostic suites can besimilarly combined to significantly enhance the diagnostic resolution.

The future of flow metering design is not solely in improving singlemeter technology concepts to get an operational advantage over competingdifferent technologies. The future of flow metering also lies withembracing the advantages of different metering technologies and usingthem together to support each other.

Various improvements and modifications can be made to the above withoutdeparting from the scope of the disclosure.

The invention claimed is:
 1. A method of analysing fluid flow,comprising: performing a first set of diagnostic checks associated witha first fluid flow meter of a first type which measures fluid flowaccording to a first physical principle, said first set of diagnosticchecks including readings or calculations for validating a fluid flowrate measured by the first flow meter; performing a second set ofdiagnostic checks associated with a second fluid flow meter of a secondtype which measures fluid flow according to a second physical principle,said second set of diagnostic checks including readings or calculationsfor validating a fluid flow rate measured by the second fluid flowmeter; and cross-referencing the first set of diagnostic checks and thesecond set of diagnostic checks to identify or provide additionalinformation about a metering malfunction.
 2. The method of claim 1,wherein cross-referencing the first set of diagnostic checks and thesecond set of diagnostic checks comprises identifying diagnostic alertsthat are common between the first set of diagnostic checks and thesecond set of diagnostic checks.
 3. The method of claim 1, wherein:performing the first set of diagnostic checks associated with the firstfluid flow meter comprises providing a first set of possible faultconditions; performing the second set of diagnostic checks associatedwith the second fluid flow meter comprises providing a second set ofpossible fault conditions; and cross-referencing the first set ofdiagnostic checks and the second set of diagnostic checks comprisesidentifying possible fault conditions which are common to the first setof possible fault conditions and the second set of possible faultconditions.
 4. The method of claim 1, wherein the first fluid flow meteris an ultrasonic fluid flow meter and the second fluid flow meter is adifferential pressure fluid flow meter.
 5. The method of claim 1,wherein the first fluid flow meter is a vortex fluid flow meter and thesecond fluid flow meter is a differential pressure fluid flow meter. 6.The method of claim 1, wherein at least one of the first set ofdiagnostic checks and the second set of diagnostic checks is associatedwith a differential pressure meter which comprises checking two or moredifferential pressure measurements taken by the differential pressuremeter, or measures derived therefrom.
 7. The method of claim 6, whereinthe differential pressure measurements are selected from the traditionaldifferential pressure, recovered differential pressure, and permanentpressure loss differential pressure, from one or more of the PLR, PRR orRPR ratios or normalised values thereof.
 8. The method of claim 6,wherein the differential pressure measurements taken by the metercomprise: a total head loss differential pressure taken between aposition upstream of a fluid obstruction and a position downstream ofthe fluid obstruction; a traditional differential pressure taken betweenthe position upstream of the fluid obstruction and an intermediateposition between the upstream and downstream positions; and a recoverydifferential pressure taken between the intermediate position and thedownstream position.
 9. The method of claim 1, wherein at least one ofthe first set of diagnostic checks and the second set of diagnosticchecks is associated with an ultrasonic flow meter which compriseschecking one or more of: the path speed of sound, velocity ratios,inclusive of one or more of Profile Factor, Symmetry and Cross Flowfactors, transducer gain, path performance, path turbulence, and pathsignal to noise ratio.
 10. The method of claim 1, wherein the first setof diagnostic checks and the second set of diagnostic sets areassociated with a vortex flow meter, and comprises checking the vortexshedding signal to the background noise, checking therepeatability/stability of the vortex shedding, and monitoring theamplitude and wave form of the shedding vortices.
 11. A computer programproduct including instructions that when executed on a computer devicecauses it to carry out calculations required for the method of claim 1.12. A flow metering apparatus, comprising: a first fluid flow meter of afirst type which is for measuring fluid flow according to a firstphysical principle; a second flow meter being of a second type which isfor measuring fluid flow according to a second physical principle; and adiagnostic system arranged to: perform a first set of diagnostic checksassociated with said first fluid flow meter, said first set ofdiagnostic checks including performing readings or calculations forvalidating a fluid flow rate measured by the first fluid flow meter;perform a second set of diagnostic checks associated with said secondfluid flow meter, said second set of diagnostic checks includingperforming readings or calculations for validating a fluid flow ratemeasured by the second fluid flow meter; and cross-reference the firstset of diagnostic checks and the second set of diagnostic checks toidentify or to provide additional information about a meteringmalfunction.
 13. The apparatus of claim 12, wherein the first fluid flowmeter is an ultrasonic flow meter and the second fluid flow meter is adifferential pressure flow meter.
 14. The apparatus of claim 13, whereinthe differential pressure flow meter comprises a Venturi constriction,and the ultrasonic flow meter comprises at least one transducer pairprovided at a throat portion of the Venturi constriction.
 15. Theapparatus of claim 12, wherein the first fluid flow meter is a vortexflow meter and the second fluid flow meter is a differential pressureflow meter.
 16. The apparatus of claim 15, wherein the differentialpressure flow meter comprises a Venturi constriction, and the vortexflow meter comprises a vortex shedding bluff body at a throat portion ofthe Venturi constriction.
 17. The apparatus of claim 16, wherein thedifferential pressure flow meter comprises a fluid obstruction elementplaced in a fluid conduit; and a support member for the fluidobstruction element is also a vortex shedding bluff body.
 18. Theapparatus of claim 17, wherein the support member extends across theentire diameter of the fluid conduit.
 19. The apparatus of claim 17,further comprising a vortex shedding sensor system positioned downstreamof the support member but upstream of a nose portion of the fluidobstruction element.
 20. The apparatus of claim 17, further comprisingpressure ports for measuring pressures, the pressure ports beingangularly spaced from a vortex sensor port at the fluid conduit.
 21. Theapparatus of claim 17, wherein the fluid conduit is provided through thesupport member for a pressure port at one end and for a vortex sensor atits opposite end.
 22. The apparatus of claim 16, wherein the fluidobstruction element comprises a cone-shaped element comprising a pair offrusto-conical portions joined at their larger ends.